The present invention generally relates to power amplifiers, and particularly to compensation for the non-linear distortion of a power amplifier having a negative feedback circuit.
Radio systems using a linear digital modulation system such as 16 QAM (Quadrature Amplitude Modulation) or π/4QPSK (Quadrature Phase Shift Keying) absolutely need the compensation for the non-linear distortion of power amplifier circuit, and employ various types of non-linear compensation system (linearizer). Of the compensation systems, the linearizer of Cartesian loop negative feedback system has been used from a long time ago. A conventional power amplifier circuit having a linear feedback circuit will be described with reference to FIG. 2. FIG. 2 is a block diagram of the construction of the transmission section (negative feedback power amplifier) of a radio equipment using the Cartesian loop negative feedback linearizer system.
A base-band signal generator 1 generates an in-phase component (I) and quadrature component (Q) of a base-band signal, and supplies them to adders 2-1, 2-2, respectively. The adder 2-1 adds the in-phase component (I) and an in-phase component (I′) of a feedback signal, and supplies the resulting in-phase component (i) to a loop filter 3-1. Similarly, the adder 2-2 adds the quadrature component (Q) and an in-phase component (Q′) of the feedback signal, and supplies the resulting quadrature component (q) to a loop filter 3-2.
The loop filter 3-1 restricts the frequency band of the input in-phase signal (i), and supplies it to a quadrature modulator 4. Similarly, the loop filter 3-2 restricts the frequency band of the input quadrature component signal (q), and supplies it to the quadrature modulator 4.
In addition, a reference signal generator 11 generates a reference frequency signal and supplies it to a PLL frequency synthesizer 12. The PLL frequency synthesizer 12 generates a carrier signal on the basis of the reference frequency signal, and supplies it to the quadrature modulator 4 and a phase shifter 13. The phase shifter 13 shifts the phase of the input carrier signal on the basis of a control signal fed from a phase controller 16, and supplies the phase-shifted carrier signal to a quadrature demodulator 15.
The quadrature modulator 4 produces a modulated signal with a desired frequency band as a result of the modulation of the carrier signal with the in-phase component (i) and quadrature component (q) of the input base-band signal, and supplies it to a band-pass filter 5 (BPF).
The band-pass filter 5 removes the unnecessary components from the modulated signal, and supplies it to a power amplifier (PA) 7. The power amplifier 7 amplifies the input signal to a specified output level, and supplies it to a directional coupler 8. The signal passed through the directional coupler 8 is supplied to a low-pass filter (LPF) 9 where the harmonic components are removed from the modulated signal, and then it is transmitted through an antenna 10.
This power amplifier circuit is constructed to have a Cartesian loop negative feedback linearizer. In other words, the directional coupler 8 supplies part of the output signal from the power amplifier 7 to a variable attenuator (ATT) 6-2. The variable attenuator 6-2 adjusts the power level of the input signal to a proper value, and supplies it to the quadrature demodulator 15. The quadrature demodulator 15 processes the input carrier signal from the phase shifter 13 and the input feedback signal from the variable attenuator 6-2 to produce the in-phase component (I′) and quadrature component (Q′) of the feedback base-band signal, and it supplies the in-phase component (I′) to the (−) negative input terminal of the adder 2-1 and the quadrature component (Q′) to the (−) negative input terminal of the adder 2-2.
Thus, the output from this power amplifier circuit is negatively fed back to the input of the power amplifier circuit.
In this negative feedback power amplifier circuit, in order to stabilize the system, it is necessary that the phases of the input signals (in-phase component (I) and quadrature component (Q)) and the corresponding feedback signals (in-phase component (I′) and quadrature component (Q′)) be coincide with each other. Therefore, the phase controller 16 controls the phase shifter 13 to shift the phase so that the phase differences between the input signals and the feedback signals are zero degrees. To this end, the quadrature component (Q) of the signal (input signal) fed from the base-band signal generator 1 to the adder 2-2, and the quadrature component (q) fed from the loop filter 3-2 to the quadrature modulator 4 are also supplied to the phase controller 16.
The adders 2-1, 2-2 work as subtracters. Alternatively, the adders 2-1, 2-2 may add the input signals to the feedback signals when the phase differences between the input signals and the feedback signals are controlled to be 180 degrees.
That is, the phase controller 16 compares the phase of the quadrature component (Q) of the input signal and the phase of the quadrature component (q) of the output signal from the adder 2-2, detects the phase deviation, and supplies to the phase shifter 13 the control signal to compensate for the detected phase deviation to a specified value. The phase shifter 13 shifts the phase of the carrier signal fed to the quadrature demodulator 15 on the basis of the input control signal. Thus, the input signals and the feedback signals at the adders 2-1, 2-2 can be adjusted in their phases so that the phase differences coincide with zero degrees.
In the phase shifter 13, in order to attain an arbitrary phase shift as shown in FIG. 5, DC voltages (x1, y1) corresponding to the orthogonal coordinates (x, y) are applied to the carrier signal fed from the PLL frequency synthesizer 12, thereby making quadrature modulation to produce a carrier signal of a given initial phase (θ). FIG. 5 is a diagram for explaining the operation of the phase shifter. The abscissa is the in-phase component of the base-band signal, and the ordinate is the quadrature component of the base-band signal. Thus, the signal state is shown by the in-phase component-quadrature component plane.
If the power supply used in the phase shifter 13 is, for example, a single power source of +5 V, the possible operation range of the carrier signal is 0 V˜5 V. In order to achieve the maximum signal dynamic range, the signal is required to be operated around the center of 2.5 V. At this time, 2.5 V is the reference voltage. Thus, the signal is operated around the reference voltage of 2.5 V. The DC voltage (control signal) fed to the phase shifter 13, in this case, is within the range of 2 V˜3 V.
Moreover, in order to keep the output signal level from the transmission section (power amplifier circuit) constant, it is necessary to adjust the gain of the feedback path. In the negative feedback power amplifier circuit, when the loop gain is much larger than 1, the output level is generally determined by the gain (amount of attenuation) of the feedback path. Therefore, the signal level in the feedback path is necessary to be kept constant in order to make the output signal level constant.
In other words, a part (feedback signal) of the output signal that is fed back through the directional coupler 8 is supplied to a detector 14. The detector 14 detects the envelope of the input signal, and supplies a voltage proportional to the envelope signal level to an A/D converter 22. The A/D converter 22 converts the detected signal from the detector 14 to digital data, and supplies it to a control portion 20.
The control portion 20 controls the variable attenuator 6-2 so that the voltage level to be detected by the detector 14 can have a specified value. In this structure, a DC voltage is controlled via a D/A converter 21 to generate a control signal. This specified value is a signal level to be detected by the detector 14 when the output from the power amplifier 7 is a certain value. This value is experimentally determined. If the output value from the power amplifier 7 is, for example, 10 W as a predetermined value, and if the normal signal level to be detected by the detector 14 is 1 V, the specified value is this value of 1 V. The control portion 20 generates the control signal that controls so that the output from the detector 14 can be maintained to be 1 V, and supplies it to the D/A converter 21. The D/A converter 21 converts the DC voltage to analog data, and supplies it to the variable attenuator 6-2, controlling the amount of attenuation in the variable attenuator 6-2. Thus, the output voltage from the detector 14 controls the gain, i.e., feedback ratio of the feedback circuit, so that the output signal level from the power amplifier 7 can be maintained constant.
The negative feedback operation will be briefly described with reference to FIG. 4. FIG. 4 is a block diagram of the construction of the basic negative feedback power amplifier circuit.
Referring to FIG. 4, an input signal (signal level: X) is supplied via an input terminal 41 to an adder 42, and the output signal from the adder 42 is fed to an amplifier 43. The amplifier 43 gives the input signal a predetermined gain (A), and supplies it (signal level: Y) to an output terminal 45. The amplifier 43 supplies a part of the output signal (signal level: Y) to a feedback path circuit 44. The feedback path 44 gives the input signal a predetermine gain (β), and supplies it to the (−) negative input terminal of the adder 42. The adder 42 subtracts the fed-back signal at the output of the feedback path 44 from the input signal at the input terminal to produce the difference signal.
At this time, the output signal level Y is given asY=A·X/(1+A·β)  (1)where (A·β) is the loop gain. If (A·β)>>1, the above equation can be expressed asY≅X/β,and thus the output level can be determined by the gain (amount of attenuation) β of the feedback path.
Therefore, the output signal level Y can be maintained constant by the control in the variable attenuator 6-2 as shown in FIG. 2.
The amplifier 43 in FIG. 4 is formed of at least the loop filters 3-1, 3-2, quadrature modulator 4, band-pass filter 5, power amplifier 7, and a part of directional coupler 8 in FIG. 2. The feedback path 44 is formed of at least a part of directional coupler 8, the variable attenuator 6-2 and quadrature demodulator 15.